Rectal bleeding is a common problem in infants. In an infant with rectal bleeding who is otherwise well, an empiric diagnosis is often made of allergic colitis. Treatment consists of changing the diet to a protein hydrolysate formula. These formulas are three times more expensive than standard infant formula. Some patients require elemental (amino acid) formulas that are four times as expensive as standard formula. There is controversy in the exclusively breast fed infant as to whether the mother should go through dietary changes or discontinue breastfeeding and begin the infant on a protein hydrolysate formula. An understanding of the pathophysiology may lead to better treatment or prevention of allergic colitis, as well as other food allergic conditions.